1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to an LCD device having an optically compensated bend (OCB) mode liquid crystal layer.
2. Description of the Related Art
An LCD device has a liquid crystal injected between a pixel electrode and an opposite electrode, and changes alignment of the liquid crystal by forming an electric field between the pixel electrode and the opposite electrode. The changed alignment of the liquid crystal controls transmittance of light to form images.
Of the various types of LCD devices, an optically compensated bend (OCB) type LCD device has a fast response speed and an excellent viewing angle. The OCB type LCD device includes a pixel electrode, a lower alignment layer, an opposite electrode, an upper alignment layer, and a liquid crystal layer having a positive dielectric constant anisotropy Δε. The lower and upper alignment layers are rubbed (and/or coated) in the same direction, so that the liquid crystals have a splay alignment.
In order to form images in the OCB type LCD device, a high electric field should be formed between the pixel electrode and the opposite electrode. The high electric field changes a tilt angle of the liquid crystals located at a central portion of the liquid crystal layer to an angle of 90 so that the liquid crystals have a bend alignment. This is referred to as a bend transition. In addition, a predetermined voltage is applied between the pixel electrode and the opposite electrode to induce a change of a tilt angle of the rest of the liquid crystals other than the liquid crystals adjacent to the alignment layers and the liquid crystals located at the central portion. Thus, polarization of light which passes through the liquid crystal layer is changed to form images.
In order for the LCD device having a plurality of pixels to implement high resolution images, most of the liquid crystals arranged in the plurality of pixels should be bend-transited. However, this requires a lot of time, and pixels which are not bend-transited may still occur. In order to prevent this, a voltage is increased for the bend transition, leading to a high power consumption.
In order to resolve the above problem, an LCD device having protrusions is disclosed in a Korean Patent Publication (No. 2001-60522). The LCD device includes upper and lower substrates (or first and second substrates) which are facing each other. First and second electrodes are respectively formed on respective inside surfaces of the first and second substrates. Protrusions are disorderedly formed at corresponding locations on each of the first and second electrodes, and alignment layers are formed to cover the protrusions on the respective electrodes. A nematic liquid crystal layer having a positive dielectric constant anisotropy is injected between the alignment layers of the two substrates. Liquid crystal molecules of the liquid crystal layer are aligned to have a predetermined angle to a surface of the substrate along a tilt angle of the protrusions by an anchoring force of the alignment layer, so that they can have a stable bend alignment. As a result, an initial transition voltage can be lowered. However, a disclination line may be generated around the protrusions. Because of this, the protrusions, which are disorderly arranged, may cause a display quality degradation.
An LCD device having a structure of promoting a propagation of transition is disclosed in a Japanese Patent Publication (No. 2002-296596). The LCD device includes an array substrate and an opposite substrate with a liquid crystal layer interposed therebetween. The structure of promoting the propagation of transition is located on either of the array and opposite substrates. The structure of promoting the propagation of transition may be an electrode wire line for propagation or a protruding portion. However, controlling alignment of the liquid crystal using only the protruding portion may cause an insufficient bend transition.
An LCD with transition nucleus areas is disclosed in a Korean Patent Publication (No. 2002-97025). The LCD device includes first and second substrates, a plurality of pixel electrodes formed on the first substrate, and a common electrode formed on the second substrate. A first transition nucleus area with a plurality of continuous slant surfaces having a saw-tooth cross-sectional profile is formed on the first substrate layer. A second transition nucleus area with a plurality of continuous slant surfaces having a saw-tooth cross-sectional profile is formed on the second substrate. However, it may be difficult to continuously form the saw-tooth cross-sectional profile in a small area, and controlling alignment of the liquid crystal by only the transition nucleus area having the saw-tooth cross-sectional profile may cause an insufficient bend transition.